This invention relates to the manufacture of electrically welded, tubular, steel products such as steel pipe having a longitudinal seam which is butt joined by electrical resistance welding, submerged arc welding or the like. Steel pipe is commonly made from heavy strip or plate of hot-rolled steel, called skelp, provided in long pieces or coiled lengths, which have their longitudinal edges finished appropriately for butt welding together when the skelp is brought into a cylindrical configuration. Such shaping of the skelp into tubular form is achieved by suitable roll means, such as successive concave rolls past which the skelp is advanced while the rolls progressively bend it about the longitudinal axis intended for the tube, or in the case of very large diameter pipe (e.g. about 25 inches or more in diameter) means such as a stand of long, heavy rolls on axes parallel to the desired pipe axis, which bend an entire length of partly bent, sidewise-received skelp into the intended shape.
In these or other ways, the skelp is brought, progressively or as a complete piece, into a cylindrical form, with a narrow, longitudinal cleft between the original skelp edges. Then further rolls or other means compress the outside of the pipe blank to close the cleft, at least as it passes or is passed by the welding means, which welds the butted edges together. For large diameter pipe, such electrical welding may be of the submerged arc type, on the outside of the cleft, with a second, subsequent weld by another consumable electrode along the inside. For a great many sizes of pipe, particularly under 25 inches, electrical resistance welding is used, by allowing electric current to pass through the abutted skelp edges successively along the cleft zone; the junction provides a high resistance, generating sufficient heat to melt the adjacent steel and form the desired weld.
Welded pipe of these types is rigorously tested along the weld zone, by non-destructive testing techniques such as using suitable radiation to detect imperfections, particularly in the welded metal. It has been found that an undesirably high percentage of pipe lengths are rejected in the tests, and that such rejections are often caused by non-metallic inclusions in the steel, such as oxide stringers resulting from deoxidation practice with aluminum or silicon, e.g. in semi-killing or killing techniques desired for other advantage in the steel. Because of such inclusions, as of aluminum oxide, and indeed because of other compositional requirements, the butted edges of steel are more difficult to weld than might be the case with simple, rimmed steel; yet strength requirements for the pipe have dictated the use of steels which have contents of carbon, manganese, silicon, or others, or are killed or grain-refined with aluminum, such that difficulty is encountered in welding the pipe, because of non-metallic, oxide inclusions, or for other known reasons. As will be understood, the main body of the pipe wall of such steels may be deemed entirely sound despite inclusions, but if the weld is imperfect, the pipe must be rejected.